In recent years the need for global data networking capability has rapidly expanded. In order to meet this need, broadband satellite communication systems have been proposed as an alternative to land-based communication systems. One type of satellite data communication system is described in a variety of U.S. patents assigned to the assignee of this patent application, including U.S. Pat. Nos. 5,386,953; 5,408,237; 5,527,001; 5,548,294; 5,641,135; 5,642,122; and 5,650,788. These patents and other pending applications assigned to the assignee of this patent application describe a satellite communication system that includes a constellation of low-Earth-orbit (LEO) satellites for transmitting data from one location on the Earth's surface to another location. More specifically, each LEO satellite has a communication "footprint" that covers a portion of the Earth's surface as a satellite passes over the Earth. The communication footprint defines the area of the Earth within which ground terminals can communicate with the satellite. Located within each footprint are a large number of cells. During the period of time a cell remains within the borders of a satellite footprint, ground terminals located in the cell transmit data to and receive data from the "servicing" satellite. When a satellite reaches the end of its servicing arc, another satellite in orbit is positioned to "service" the Earth-fixed cells previously covered by the satellite reaching the end of its servicing arc.
Data to be sent from one location on the Earth to another location is transmitted from a ground terminal located within a cell to the satellite serving the cell via an uplink data channel. The data is routed through the constellation of LEO satellites via an intersatellite link data channel to the satellite serving the cell within which the ground terminal of the designated receiver is located. The latter satellite transmits the data to the receiver ground terminal via a downlink data channel. Thus, the constellation of LEO satellites and the ground terminals form a satellite data communication network wherein each ground terminal and satellite forms a node of the network.
Typically, data transmissions sent via uplink, intersatellite link or downlink data channels are broken into digital data packets, each of which include a header and a payload. The header data packets contain address and control information designed to direct the data packets through the satellite constellation to a desired ground terminal. The payload contains the information being transmitted, which is intended for the satellite or the ground terminal or both.
In order for a LEO satellite data communication system to be competitive, it must allow broadband transmission at a relatively low cost. Low cost requires that the satellites be light in weight and relatively inexpensive to manufacture. One way of keeping satellite weight and cost low is to minimize the complexity of electronic signal processing hardware, and keep transmission and reception power requirements low. Unfortunately, low transmission and reception power conflicts with the need for a highly reliable data communication system having a low bit error rate and packet loss rate as it is relatively easy to lose data contained in low-power signals.
One way of improving the reliability of low-power data communication signals that is well-known in the satellite communication field is to forward error correction (FEC) code the data to be transmitted. See U.S. Pat. Nos. 5,117,427; 5,446,747; and 5,473,601 for examples of FEC coding of digital data signals. Other systems and methods which use FEC coding in novel ways to mininize power requirements and maximize reliability are described in U.S. patent application Ser. No. 09/035,645 entitled "Coding System and Method For Low-Earth-Orbit Satellite Data Communication" and U.S. patent application Ser. No. 08/949,412 also entitled "Coding System and Method For Low-Earth-Orbit Satellite Data Communication", both assigned to the assignee of this patent application and both incorporated herein by reference.
The present invention is directed to a coding system for a LEO satellite data communication network that uses FEC coding in a novel way not only to minimize power requirements and maximize reliability but also to minimize the bandwidth used for coding.